1. Field of the Invention
The present invention relates to testing of integrated circuits and more particularly concerns improved test fixtures and probes that readily lend themselves to automated testing and efficient probe manufacture and use.
2. Description of the Related Art
Integrated circuits or chips including conductive traces, electrical components, and active devices are fabricated in batches of large numbers of similar or identical circuits on a single wafer and then individually cut from the wafer for use. Because production techniques and processes are pushed to the limits of accuracy and repeatability, significant numbers are of circuits on a single wafer may be unacceptable or inoperable. Therefore it is desirable to test each circuit individually before it is separated from the other by severing the wafer into its many component circuits for its intended use.
Probe cards presently employed for testing of integrated circuits while still on the wafer employ a number of probe contact elements, commonly in the form of very small blades or needles that are mechanically and electrically fixed to a circuit board or the like and have leads that fan out to outer edges of the probe card for making connections between the probe card and testing circuitry. The contact blades or needles of the probe card are moved into contact with specific areas, namely the pads of the integrated circuit or chip under test, and may be electrically connected so as to apply selected input signals and to read output signals from the device under test. In this manner the chips are tested on the wafer, before being connected for intended use, by applying operating signals and monitoring and evaluating resultant outputs.
Probe cards presently employed are bulky and complex, being difficult to store and handle. Mechanical contact needles and blades require precise alignment and positioning. They also require precise planarization. That is, the contact surfaces of the probe must all lie in the same plane. An example of such a probe card employing a number of small, thin metal blades having needle like probe members fixed thereto is illustrated in U.S. Pat. No. 4,161,692 of Tarzwell for Probe Device For Integrated Circuit Wafers. In probe cards of this type the individual probe blades or needles must be mounted individually, frequently by use of micro-manipulators to properly locate the closely packed small probe elements, which then may be soldered or otherwise fixed in place. Final position adjustment in both X,Y, and Z, that is, in lateral position and elevation, is then accomplished for individual ones of the probe blades and needles by bending. For planarization, the probe card may be brought down upon a flat plate so as to ensure that all of the probe contacts will touch at the same time. These procedures are time consuming, tedious and expensive. Moreover, because the final positioning of contacts of prior probe cards is accomplished by bending of the metal blades or needles, the device is subject to errors caused by creep. That is, the parts, after bending, tend to "creep" or return to an original condition or position in which they existed prior to being stressed during the bending adjustment process. The creep occurs even when the probe card is on the shelf and not being used, so that, after a period of months or sometimes weeks on the shelf, the probe contacts may need to be readjusted. More frequent adjustment and probe maintenance may be required when the probes are used.
In use of such probe cards the contact between the probe card blades or needles and the circuit chip is frequently made by a scrubbing action, which tends to deflect the slender probe elements as much as several mils on each touchdown. This displacement, which occurs repetitively upon each test, further tends to change the desired positioning of the elements.
The probe may have from fifty to several hundred contacts, each of which must be precisely and individually positioned with respect to all others so that upon contact with the circuit chip all probe contacts will contact all pads of the chip under test. All of this means that the probe cards presently used are exceedingly expensive, require much maintenance, and are subject to many errors.
Other probes employing blades and needles are illustrated U.S. Pat. No. 4,783,625 to Emory J. Harry et al, and U.S. Pat. No. 4,791,363 to John K. Logan. U.S. Pat. Nos. 4,849,689 to Gleason and 4,697,143 to Lockwood show cantilevered trapezoidal probes which may include detachable tip sections and circuit boards that mount probe conductors.
As integrated circuit speeds increase, so too do the difficulty and extent of the testing problems. Such problems are caused by cross talk between adjacent signal traces, signal loss and degradation due to capacitative loading of the circuit under test, and increased need for shielding and impedance matching of signal lines throughout the test fixture and test head. High frequency probe transmission lines must be properly terminated. Although high frequency hardware can be provided up to the test head itself, the physical connection between the test head and the integrated circuit pad, which relies upon exposed metallic blades or needles, provides poor high frequency performance and extremely fragile components. Thus the final part of the probe transmission line cannot be properly terminated.
It is often necessary to increase the density of probe contact pads and to provide test contact with chip pads that are located at the interior of the chip. This may require that leads to the probe contacts cross one another or that the blades or needles of the existing probe cards cross one another. Such a crossing of blades or needles is not physically possible with present configurations of test probes.
As size and spacing of integrated circuit chips decrease and density of chip contacts increases, it becomes ever more difficult to make mechanical blade or needle contacts as small and closely spaced as required for proper testing of modern integrated circuit chips. Moreover, the great bulk and complex configuration of prior art probe cards do not readily lend themselves to automated handling or simplified storage. Frequently the relatively costly test fixtures themselves must be changed whenever the probe card is changed for testing of a different chip configuration.
The test probe described in the above-identified co-pending patent application of Kamensky et al employs photolithographically formed probe contacts and leads to eliminate many of these problems but fails to provide for a number of improvements that are available with the methods and apparatus described herein. The disclosure of such co-pending application is incorporated by this reference as though fully set forth herein.
Accordingly, it is an object of the present invention to provide for testing of integrated circuit chips while avoiding or minimizing above-mentioned problems.